Actually you can't BUT
I'll assume you've got a regular ipod and windows.
When yuo first connected the ipod to the first PC the ipod called that PC 'Home." Each time you connect it will download the library on it automatically.
When you connect the iPod to a different PC, that ipod asks for permission before synchronizing with this music library. If you choose Auto-sync, your ipod recognizes the second PC as its home PC and breaks the original PC link. If you do not choose Auto-sync, the ipod switches to manual update mode, allowing you to select and drag songs, albums, or playlists from the second PC to your ipod.
BUT you can copy it with your ipod this is the how to from apple
Synchronization occurs only in one direction, from your computer to your iPod. This means you cannot transfer music, automatically or manually, from your iPod to a computer, and you cannot use iPod to copy a music library from one computer to another. But, if you legally are allowed to copy music files, you can configure your iPod as a hard disk to move music files. Click here